It is known from the U.S. Pat. Nos. 5,552,261 and 5,529,473 that the diffusion of monomers having an increased or reduced refractive index compared to a surrounding liquid matrix can be used for producing a refractive index gradient. This effect, which is known in the case of polymers as the Colburn-Haines effect, can lead after subsequent polymerization to a product having a refractive index gradient. Such polymers are also referred to as photopolymers.
U.S. Pat. Nos. 3,658,526, 4,959,284, 4,942,112, 5,013,632, 5,098,803 (E.I. Du Pont Nemours) describe the preparation of photopolymers by free-radical polymerization of monomers in a solid medium. This not only requires the exclusion of oxygen, but severe shrinkage of the material also occurs as a result of the polymerization. Moreover, the addition of a solvent is necessary for the coating of surfaces. The thickness of the material is therefore restricted to up to 150 μm.
U.S. Pat. No. 6,482,551 describes the use of a matrix which is polymerized in a first step. The monomers of the photopolymer are polymerized only in a second step to form the actual photopolymer. Since the mixture of the monomers and the matrix can be selected so as to be liquid, no addition of a solvent is necessary for the coating of surfaces and coatings having a thickness of over some millimeters can be produced. The monomers of the photopolymer and of the matrix must not react with one another in any of the steps. Thus, U.S. Pat. No. 5,453,340 uses cationically polymerizable monomers such as epoxides or oxetanes which can be polymerized using Bronsted or Lewis acids for the matrix and free-radically polymerizable monomers for the photopolymers. This again leads to high shrinkage in the illuminated regions. In addition, the material also has to be illuminated twice with different wavelengths.
In the patent applications US 2003/0157414 and US 2005/0231773, epoxides, for example, are used as monomers for the photopolymer in order to reduce shrinkage. However, these compositions require a higher energy for writing of the optical information, i.e. on illumination, than the above-described materials which are based on free-radical polymerization.
U.S. Pat. Nos. 7,022,392 and 6,969,578 are concerned not only with a polymerization of monomers but also with photoisomerization of individual molecules which leads to a local change in the refractive index. Only small changes in illumination therefore occur in the case of these. However, this system requires special monomers.
Experiments aimed at providing a photopolymer in an inorganic-organic matrix are described in the U.S. Pat. Nos. 6,268,089 and 7,163,769. The matrix is produced in a sol-gel process and the photopolymer is polymerized both free-radically and cationically. The materials obtained do not display any great advantages over the previously known materials.
Another way of producing gradient structures is described by the patent application US 2005/0101698. Here, a concentration gradient of nanoparticles is produced in a composite and fixed by polymerization of the matrix surrounding these. This process makes it possible to produce volume holograms having a refractive index of 90%. However, the thickness of the layers is limited and the material displays high shrinkage because of the free-radical polymerization.
In summary, the development of photopolymers has made great progress in recent years, but the known systems still have some disadvantages. Thus, the systems used are not sensitive enough to allow very sharp modulation of the refractive index. Here, the sensitivity is in terms of not only the light intensity required for polymerization but also the illumination time required for producing the gradient structure. Both limit the resolution and refractive efficiency of the gradient structures produced. At the same time, the bandwidth of refractive index modulation achieved in the material also plays an important role. Both parameters mentioned limit, for example, the minimum layer thickness of the material at which the production of optical gradient structures is still possible.
Since optical gradient structures are nowadays used in many fields having extremely different requirements, for example in respect of mechanical flexibility, thickness and stability, a high variability of the components used is of great importance.